The basis for the association between Mac-1 and complement receptor type three (CR3) function will be investigated. The hypothesis that Mac-1 is the CR3 will be tested by determining whether purified Mac-1 can interact with C3bi-coated particles. Further MAb binding to different topographic regions and extracellular and intracellular domains of Mac-1 will be prepared. The location of the C3bi binding site will be probed by comparing the inhibitory effect of MAb with their specificity for subunits (alpha or beta) and proteolytic fragments of Mac-l. Binding of Mac-1 fragments to C3bi-coated particles will be tested. Whether Mac-1 is a receptor which is recycled will be investigated. The steady state ratio of extracellular/intracellular Mac-1 will be determined. Changes in Mac-l structure or modification occurring after ligand binding will be examined. The orientation and arrangement of Mac-1 in the membrane, and the size of intracellular, extracellular, and intramembranous domains will be investigated. The location of proteolytic fragments will be determined by vectorial labeling and reactivity with MAb to different topographic regions and extracellular and intracellular domains. The structural basis of the homology between Mac-1 and LFA-1, an antigen associated with T lymphoctye-mediated killing, will be further investigated. Mac-1 will be purified and fragments subjected to amino acid sequencing. N-terminal sequencing has shown that the alpha subunits are 33% homologous. (CS)